Increase in Si:N drawdown ratio due to resting spore formation by spring bloom-forming diatoms under Fe- and N-limited conditions in the Oyashio region

Resting spore formation and Si:N drawdown ratios were investigated under iron (Fe)- and nitrogen (N)-limited conditions using a unialgal culture of Thalassiosira nordenskioeldii and natural phytoplankton assemblages during the spring bloom in the Oyashio region. In the unialgal culture of T. nordenskioeldii, 20% and 100% of the cells formed resting spores under Fe- and N-limited conditions, respectively. The Si:N drawdown ratios were 2- and 14-fold higher in Fe- and N-limited conditions, respectively, compared to Fe- and N-sufficient conditions. At the start of the natural phytoplankton incubation, 18 among 47 identified diatom species were known resting spore-forming species. Approximately 15 common diatom species formed resting spores under Fe- and N-limited conditions. During the natural phytoplankton incubation, the percentage of the resting spores increased with time under both Fe- and N-limited conditions, reaching 25% and 40% of total diatom abundance, respectively. The Si:N drawdown ratios significantly increased with an increase in the contribution of resting spores in both the unialgal culture and natural phytoplankton incubations. These results suggest that if the bloom dominated by neritic, resting spore-forming diatom species decline by either Fe- or N-depletion, Si may be utilized preferentially to N in the upper mixed layer due to the formation of heavily silicified resting spores.     

Differences in cell viabilities of phytoplankton between spring and late summer in the northwest Pacific Ocean

Cell viabilities of phytoplankton in the Oyashio and Kuroshio-Oyashio transition regions of the northwest Pacific Ocean were examined in September 2003 (late summer) and May 2005 (spring) using a membrane permeability test. Specific lysis rates of the phytoplankton during late summer were also assessed by an esterase activity assay. In late summer, cyanobacteria Synechococcus spp. were > 2 × 10⁴ cells ml^− 1 and numerically dominated the phytoplankton communities. The cell viabilities of Synechococcus spp. and eukaryotic ultraphytoplankton (< 10 μm in size) were 60-79% and 26-41% in surface waters, respectively. The specific lysis rates of the phytoplankton were 0.12-0.67 d^− 1 in late summer. By contrast, in spring, eukaryotic cells were predominant in the phytoplankton communities. The cell viabilities of surface eukaryotic ultraphytoplankton in spring were > 70% and significantly higher than those in late summer. During spring, Synechococcus spp. only occurred with < 1 × 10⁴ cells ml^− 1 in the Kuroshio-Oyashio transition region, and their viabilities were 80%. In the Oyashio region where a spring diatom bloom developed, the viability of fucoxanthin-containing algae (mainly diatoms and prymnesiophytes) was ca. 90%. These results suggested that the cell viability of phytoplankton could vary seasonally with their community structure in the study area. The phytoplankton cell death in late summer was particularly significant for their loss process and could support the microbial food webs by supplying dissolved organic carbon (DOC) derived from the dead cells.     

Geographical shift of zooplankton communities and decadal dynamics of the Kuroshio–Oyashio currents in the western North Pacific

We investigated temporal and spatial variations in the zooplankton community structure in the Oyashio and Transition region of the subarctic western North Pacific from 1960 to 1999 using principal component analysis (PCA) and zooplankton samples from the historical Odate Collection. In particular, we examined the influence of Kuroshio and Oyashio decadal dynamics on geographical variations in the zooplankton community. The first principal component (PC1) closely represented the interannual variation in cold-water, large copepod species, while the second PC (PC2) represented the variation in warm-water, small copepod species. Using Rodionov's regime-shift method, we detected a significant increase in the PC score after 1976 and 1981 for PC1 and PC2, respectively. After the shift years, (1) warm-water species increased in the Transition zone, (2) the distribution center of the cold-water species shifted southward, and (3) copepod abundance and species diversity increased in the Transition zone as a result of (1) and (2). The timing of these shifts in the zooplankton community roughly coincided with the North Pacific climatic regime shift in 1976/1977. From the mid-1970s to the early 1980s, the southern boundary of the Oyashio shifted southward and increased geostrophic transport was observed in the Kuroshio, indicating spin up of the Kuroshio–Oyashio system. Change in atmospheric circulation during the 1976/1977 regime shift is thought to have caused the spin up of these currents, which subsequently affected the regional zooplankton community through advective processes.     

Paleoceanographic history around the Tsugaru Strait between the Japan Sea and the Northwest Pacific Ocean since 30 cal kyr BP

The paleoceanographic history around the Tsugaru Strait since 30.0 cal kyr BP was reconstructed using ecological and biogeographical habitats of diatom species and Q-mode factor analysis of diatom flora in six piston cores. At 30.0-17.5 cal kyr BP, a fall in sea level and the intensification of the Oyashio Current caused erosion of submarine sediments near the shore in the eastern area of the Strait and led to deposition of the third and second sequences of thinly laminated layers (TL3 and TL2) in the Japan Sea. At 17.5-11.5 cal kyr BP, cooling intensified around the Strait, producing an inflow of Oyashio water into the Japan Sea through the Strait. At 15.5-14.5 cal kyr BP, a cold-water mass around the Strait developed from subarctic sea-ice to arctic waters. At 13.0-11.2 cal kyr BP, the Younger Dryas cooling was clearly recorded by the increasing of oceanic cold-water diatom species in the eastern area of the Tsugaru Strait but not in the Japan Sea. At 15.5-13.0 cal kyr BP and 11.25-10.25 cal kyr BP, the warming around the Younger Dryas resulted in the deposition of thinly laminated layers off Shimokita in the eastern area of the Strait. At 9.5 cal kyr BP, the Oyashio Current water intruded into deep depths in the mixed water region off Sanriku. At 9.5-7.0 cal kyr BP, the fluctuation of 1.5-kyr intervals in the strength of the Tsushima Warm Current system in the Japan Sea was initiated.